Doubly regular asymmetric digraphs

Classification of Regular Digraphs, Normally Regular Digraphs, and Strongly Regular Digraphs

We use an exhaustive generation with isomorph-rejection to classify three types of structured digraphs. The first type is the class of regular digraphs where each vertex has the same number of out-neighbors and inneighbors. The second type is the class of normally regular digraphs introduced by Jørgensen. In these digraphs, the number of common out-neighbors (or inneighbors) of vertices x and y...

Normally Regular Digraphs

A normally regular digraph with parameters (v, k, λ, μ) is a directed graph on v vertices whose adjacency matrix A satisfies the equation AAt = kI + λ(A + At) + μ(J − I − A − At). This means that every vertex has out-degree k, a pair of nonadjacent vertices have μ common out-neighbours, a pair of vertices connected by an edge in one direction have λ common out-neighbours and a pair of vertices ...

Weakly Distance-regular Digraphs

Exponents of 2-regular digraphs

A digraph G is called primitive if for some positive integer k, there is a walk of length exactly k from each vertex u to each vertex v (possibly u again). If G is primitive, the smallest such k is called the exponent of G, denoted by exp(G). A digraph G is said to be r-regular if each vertex in G has outdegree and indegree exactly r. It is proved that if G is a primitive 2-regular digraph with...

The genus distributions of 4-regular digraphs

An embedding of an Eulerian digraph in orientable surfaces was introduced by Bonnington et al. They gave some problems which need to be further studied. One of them is whether the embedding distribution of an embeddable digraph is always unimodal. In this paper, we first introduce the method of how to determine the faces and antifaces from a given rotation scheme of a digraph. The genus distrib...